The present invention relates to a solar radiation sensor, and also to a system including such a sensor for measuring solar radiation distribution. The sensor and system of the present invention may be used for determining the available solar energy radiation in a number of possible applications, as will be described more fully below.
The most commonly used instruments for determining the available solar energy radiation include: pyrheliometers for measuring the direct radiation from the solar disk; pyranometers for measuring the global (combination of direct solar radiation and diffuse radiation) from the sky; and duration-of-sunshine instruments for measuring the fraction of daylight hours the sun is not obscured by clouds. The radiation sensors used in these instruments may be either thermal detectors or photodetectors. The global radiation measurements at meteorological stations are on horizontal surfaces. The same pyranometer, with the addition of a shading ring, is also used to measure diffuse radiation on horizontal surfaces. Not all solar radiation stations have all the above instruments, and therefore some radiation quantities are not measured.
The measured diffuse radiation is an average value, and an assumption is made about the isotropy of the sky. However, partly cloudy skies (and even clear skies) deviate from isotropic conditions. The above assumption can therefore be a source of considerable error in the design of static solar systems at tilted angles.
Knowledge of the amount of solar radiation incident at any time on a solar collector is essential for the design of economically competitive collectors and for making efficient use of energy conversion systems. Unfortunately, data routinely collected at meteorological radiation stations are not sufficient for solar energy conversion uses. Meteorologists and solar radiation scientists need radiation data in their studies of the physics of the atmosphere and in long term forecasting of radiation trends. Solar applications require knowledge of insolation on tilted surfaces for flat plate collectors, and knowledge of the angular distribution of the diffuse radiation is necessary to evaluate properly the performance of concentrating systems while comparing them with non-concentrating systems. Although insulation data on various tilt and azimuth angles can be essentially accomplished with the existing instruments, such a station would be very expensive to set up.